The present invention relates to a method of encoding color image information and, more particularly, to a method of encoding color image information which is suitable for efficiently transmitting and storing color image information.
Red, green and blue (RGB) signals, cyan, magenta and yellow (CMY) signals or YIQ signals which are used in a standard television system (NTSC system) have been used as chrominance signals to represent a color image.
The RGB signals are based on a spectrographic characteristic of a visual sense of human beings. Any color which can be visually sensed can be matched by a combination of the three colors R, G and B.
The CMY signals are derived as complementary colors of the RGB signals and are three-primary color signals which are utilized in printing for reproducing a color image by a subtractive combination of colors.
The YIQ signals were designed to reduce a transmission band of the chrominance signals in a television and they are calculated in accordance with the following formula. ##EQU1##
However, those chrominance signals were not designed to quantize the color image information with several-bit data width (code word length), and hence the changes in the chrominance signals do not uniformly correspond to changes in the colors visible to humans.
For example, in a color space having coordinate axes of three chrominance signals R, G and B shown in FIG. 1, the change of color caused by the change of the chrominance signal of the same magnitude may be a significant visible color change or may be a substantially invisible color change.
This is true for the CMY signals and the YIQ signals. In any case, the variations in the chrominance signals do not uniformly correspond to the variations in the visible colors.
As a result, when the chrominance signals are to be quantized by the several-bit code length, the changes in the colors caused by the changes of the chrominance signals corresponding to the quantization steps have large variations. Thus, the color reproducibility is lowered and the redundancy of the image information increases.
The RGB signals, CMY signals and YIQ signals do not have clear correspondence to hue, value and chroma (H, V and C) which are defined in the Munsell color notation system. Accordingly, even if only one of the three chrominance signals is changed, all of the H, V and C of the color may be affected.
As a result, when the chrominance signals are coarsely quantized by several-bit code words, each of the H, V and C of the color is affected if a rounding error as a result of signal processing or noise superposition is included.
One known encoding method of a multi-gradation image signal is a prediction encoding method which utilizes gradation correlation between adjacent pixel. An encoding method of a monochromatic multi-gradation image by the prediction encoding method will now be explained.
Referring to FIG. 2, it is assumed that a pixel X.sub.0 under consideration which is to be encoded and encoded reference pixels X.sub.1, X.sub.2 and X.sub.3 adjacent to the pixel X.sub.0 under consideration have gradations x.sub.0, x.sub.1, x.sub.2 and x.sub.3, respectively. Since the pixel under consideration and the reference pixels are close to each other on an image screen, it is anticipated that the gradation correlation among the pixels is high.
Thus, a prediction gradation x.sub.0 which is a prediction (e.g., estimation) of the gradation x.sub.0 of the pixel X.sub.0 under consideration can be calculated by utilizing the gradation correlation of the pixels and using a linear formula (2). EQU x.sub.0 =1/2(x.sub.1 +1/2(x.sub.2 +x.sub.3) (2)
A difference between the actual gradation x.sub.0 and the prediction gradation x.sub.0 is defined as a prediction error e. EQU e=x.sub.0 -x.sub.0 ( 3)
As described above, the gray-level image has the high gradation correlation between the adjacent pixels. Accordingly, it is anticipated that a probability of occurrence of the prediction error e shown by the formula (3) distributes with a peak at 0 as shown in FIG. 3.
Thus, by assigning a shorter code word to a pixel having a higher probability of prediction error e and a longer code word to a pixel having a lower probability of prediction error, the redundancy included in the image information can be suppressed and efficient encoding is attained.
In order to apply the prior art encoding method developed for the monochromatic image to a color image, the prior art encoding method developed for the monochromatic image is applied to each of the three chrominance signals. This technique is disclosed, for example, in an article "The Perceptual Color Space of Digital Image Display Terminals" by Antonio Santisteban, IBM J. RES. DEVELOP. Vol. 27, No. 2, March 1983.